Prior Art
The present invention relates to the reduction of oxides of nitrogen (NO and NO.sub.2) or NO.sub.x emissions in atmospheric type blue flame burners and, more particularly, to the provision and use of a flame insert which is positioned within the flames of the burner to regulate and reduce such emissions. The flame insert is designed to accommodate variations in the burner flames during combustion without loss of effectiveness in reducing NO.sub.x emissions or without reducing appliance efficiency. The flame insert may be combined with a secondary air baffle to achieve acceptable NO.sub.x and CO emission levels.
The atmospheric type burners of concern herein are often incorporated in gas-fired appliances. A gaseous hydrocarbon such as natural gas (methane) or bottled gas (propane) may be used as a fuel gas in such appliances. Examples of such appliances include furnaces, boilers, water heaters, room heaters, space heaters, duct furnaces, commercial cooking equipment, and ranges. Gas-fired appliances are not considered to be major sources of air pollution. However, there are areas within the United States that have very stringent emission regulations, most particularly for NO.sub.x limitations with which gas appliances must comply. Some gas appliances cannot meet these NO.sub.x restrictions.
There are many combustion and/or burner modification techniques which can reduce the NO.sub.x emission of gas-fired burners. These techniques include flue gas recirculation, staged combustion, ultra-high aeration, and burner design or redesign, as well as flame inserts and secondary air baffling, which are of particular interest herein.
Each of the foregoing techniques contemplates manipulating the time, temperature, and reactant concentration during combustion to minimize NO.sub.x formation in a flame. The achievement of the following objectives is considered necessary to lower NO.sub.x emission from a given burner: reduction in peak flame temperature, reduction in time of peak flame duration, increase in NO retention time in the decay temperature region, and reduction in the concentration of reactants in the high-temperature zone.
Various of the foregoing techniques have been applied to both gas-fired industrial burners and, to a lesser extent, residential burners. In substantially all cases, NO.sub.x reduction is possible, though at a cost of complete burner redesign in some cases. The most cost-effective prior art approach known to applicants comprises the use of a metallic screen flame insert to reduce peak temperatures.
The prior art metallic screens are typically fine mesh (5.times.5, 6.times.6 mesh per inch) and formed of conventional sized wire stock having a diameter in the range of several hundredths of an inch. Such flame inserts are positioned and shaped so that the screen is in as much of the flame as possible and becomes incandescent for purposes of radiating heat away from the flame. Typically, such screens are positioned just downstream from the inner cone of the flame at what is generally considered to be the flame hot spot.